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ColleqE of Pure anc! AppliEd Sconces 


Final Report 

NASA Planetology Grant NSG 7420 
THICKNESS OF WESTERN MARE BASALTS 


Rene A. DeHon 
Department of Geosciences 

July 1979 

^ (NASfl-CR-1 58784) THICKNESS Of WESTERN HARE N79-28101 0 

BASALTS Final "Report (University of . : ' 

Northeast Louisiana, Honroe.) 65 p . 

HC A04/HF A01 CSCE 0 3B ■ . ./ ' tTnclas 

^ ' ’ , • ,-G3/91 ",29283 1 

Northeast LouisiANA UNivERsiiy 
Monroe, LouisiANA 71209 






NASA PLANETOLOGY GRANT NSG 7420 


Thickness of the Western Hare Basalts 

Rene A. De Hon 


Department of Geosciences 
Northeast Louisiana University 
Monroe, LA 71209 


July 15, 1979 



ABSTRACT 


An isopach map of the basalt thickness in the western 
mare basins is constructed from measurements of the 
exposed external rim height of partially buried craters. 
The data, although numerically sparse, is sufficiently 
distributed to yield gross thickness variations. The 
average basalt thickness in Oceanus Procellarum and 
adjacent regions is 400 m with local lenses in excess of 
1500 m in the circular maria. The total volume of basalt 
in the western maria is estimated to be in the range of 
1.5 x 10 6 km 3 . 

Oceanus Procellarum and the western maria are largely 
composed of contiguous and superposed circular basins. 

The youngest basins are readily identified by a large 
number of basin-related features. Older basins retain 
progressively fewer identifying features. Mare Imbrium, 
Mare Humorum, and Mare Vaporum, and Sinus Medii are among 
those basins which retain a circular outline, Mare 
Cognitum and Mare Nubium are composite structures of 
contiguous basins, as is the Flamsteed-Reiner axis of 
thick basalts in western Oceanus Procellarum, Thick 
basalt lenses suggest the presence of older, nearly 
obliterated basins in the Stadius-Sinus Aestuum region 
as well as in east Oceanus Procellarum adjacent to 
southwest Mare Imbrium. 



The correlations noted between basalt surface 
features and basalt thickness in the eastern maria prevail 
in the western maria. Positive gravity anomalies are 
associated with most thick basalt discs. Mare ridges 
are located at the sites of buried topographic rises or 
in zones of transition between thin and thick basalts. 

Mare domes are located on relatively thin basalts 
associated with regional rises of the basement topography. 
Regional variations of basalt surface elevations mimic 
the subsurface relief. Most rilles are located in thin 
basalts parallel or subparallel to zones of equal thick- 
ness; however, many rilles cut mare material and terra 
with nearly equal development. 

The chief distinction between the eastern and western 
maria appears to be one of basalt volumes erupted to the 
surface. Maximum volumes of basalt are deposited west of 
the central highlands and flood subjacent terrain to a 
greater extent than on the east. The surface structures 
of the western maria reflect the probability of a greater 
degree of isostatic response to a larger surface loading 
by the greater accumulation of mare basalt. 



INTRODUCTION 


Mare basalts comprise a significant,, but not over- 
whelming portion of the lunar surface. Basalt covers 
approximately 20 percent of the surface , but the total 
volume is less than 1 percent of the total volume of the 
lunar crust (Head, 1975; .) . Accurate know- 

ledge of the basalt thickness and distribution yields 
important constraints to several types of lunar models. 
Knowledge of the thickness of mare basalts is required 
to adequately reconstruct pre-flooding surface topo- 
graphy and to establish basalt volumes. The volume of 
basalt produced during the episode of mare-filling 
provides important constraints to the lunar thermal 
history. Establishment of the basalt thickness distri- 
bution provides additional parameters in models of 
lunar gravity and isostasy, paleomagnetic correlations; 

impact geochemical mixing. In addition, correlation 
of regional topographic variations and paletopography 
supply data relevant to the origin of basalt surface 
structures . 

In previous studies it was shown that the mare 
basalts in the eastern maria average 200 - 400 m thick 
with basalt lenses near 1200 - 1500 m in the deeper 
parts of the basins. This study extends the thickness 


1 



observations to the western maria. Incontrast to the ^ 
distinct , isolated mare basins of the eastern maria, 
the basalts of the western maria merge to form a contin- 
uous surface. The large surface area with less confining 
contacts and significantly less accurate data compound 
the problem of accurate isopachus construction. 

This paper centers on the determination of the 
thickness of mare basalts and the configuration of the 
western mare basins. An isopach map is prepared for 
the region between the limits of 45°N to 45°S and 10°E to 
90°W. In addition, a synthesis of regional data, includ- 
ing model paleotopography and the location of basalt 
surface features is prepared for correlation with the 
isopach map. This paper is chiefly limited to a 
discussion of the origins of the data sets, limitations 
of the data, and presentation of the various map 
products. Important correlations between data sets are 


noted. 



DATA SOURCE AND METHODS 


METHOD 

An isopach. map has been constructed for the western 
mare basalts using the crater geometric technique which 
is on the exposed rim height of craters partially buried 
by mare basalt (Eggleton, 1963'; De Hon, 1974, 1975) . 

The technique is briefly reviewed in order to note the 
inherent limitations and generalizations of the method. 

Fresh impact craters on the moon display diameter- 
dependent morphological parameters which may be used to 
reconstruct the original morphology of buried or other- 
wise deformed craters. Important' in the estimation of 
basalt thickness is the relationship between crater 
diameter (D) and external rim height (h) . The relation- 
ship is defined by Pike (1974, 1977) as follows: 

(1) h=0.036 d!* 014 f or craters<15 km in diameter 

(2) h=0.399 d°’ 23 6 f or Cra ters-715 km in diameter 

For partially buried craters, the average diameter 

and height of the exposed portion of the rim are deter- 
mined from existing topographic maps or by shadow length 
measurements. The original rim height is determined 
from the crater diameter and the appropriate equation 
(Fig. 1) . The original rim height minus the exposed 


3 



rim height is assumed to be the thickness of material 
superposed in the vicinity of the crater (Fig. 2) . 

Pike's equations are derived from averaging 
variations at single craters. In like manner, two to 
four exposed rim height measurements were determined 
for buried craters. Thus the average exposed rim is 
used in the final thickness estimation. 

For most craters, a single thickness estimation is 
recorded for each crater. For some large craters (*7 75 
km diameter) , more than one thickness value may be 
determined for widely separated points along the rim. 

For those craters which occur along the edge of the 
basin, the thickness estimate is averaged over the 
flooded portion of the rim and plotted on the rim crest 
projection into the mare. 

Thickness estimates ignore local lensing within the 
crater interior as well as the wedging— out of mare- 
material on the outer crater rim rampart. The thickness 
estimate is thus a generalization of the average basalt 
thickness within the region of the crater without regard 
to local variations caused by the buried relief of the 
crater (Fig. 2) . The resulting thickness estimates 
(Fig. 3) are contoured to produce an isopach map. The 
isopach map (Fig. 4) portrays regional trends and 



5 

variations in basalt thickness but does not retain 
local variations due to the existence of individual 
buried craters. 

The isopach map. (Fig. 4) is constructed from 142 
interior thickness estimates and 148 points to define 
the limits of the mare-material and major intrabasinal, 
nonfDLooded terrains. The data distribution is an uneven 
scatter of points ranging from 1 point per 1° radius to 
1 point per 20° radius. Thus the precision of contouring 
is variable over different regions of the map. 

A preliminary isopach map was produced with a 
contouring program using a 5° search radius around each 
point to construct a 1° grid of the data.- A seven 
point, 2 nd order polynomial smoothing function was used 
to contour the gridded data. Some areas of the map 
could not be adequately constructed by this program; 
hence, hand-contouring was required to adjust isopach 
lines to some sparse regional data. The final isopach 
map was hand-contoured on the 1:5,000,000 mercator 
base map. An attempt was made to honor all data points; 
however, a few points could not be included in the final 
draft and some isolated data points may impose signifi- 
cant but dubious features on the map. Trend surface 
analysis up to the 8th degree failed to provide a 
significant match of the data. Harmonic surface 



analysis may provide an acceptable display of the larger 
trends and features of the region , but was not attempted 
for this study. 



7 


LIMITATIONS AND ACCURACY 

The accuracy of the isopach map is dependent on 
several factors, not all of which can be fully known or 
controlled. The following are optimum conditions: 

1. Data spacing and density are sufficient to 
construct isopach lines; 

2. Relief data at crater rims are accurate; 

3. Buried craters maintain the morphology of 
fresh craters; 

4. Buried craters selected for thickness measure- 
ments are superposed on the basin floor beneath mare 
material . 

Partially buried craters which comprise the data 
population are necessarily scattered with a random 
distribution and variable density of data points. 
Inasmuch as craters occupy large areas (10-100 km in 
diameter) , the thickness estimate at any one crater 

i 

represents an average thickness over a substantially 
larger area. Further, the number of partially buried 
craters preserved in a region is a function of the 
original crater density in the area, crater size, and 
thickness of superposed materials. Large numbers of 
small craters of low relief are lost by total submer- 
gence (burial) while only the larger craters are 



8 


preserved. Hence , as the thickness of the superposed 
materials increases, the point density of data 
decreases. As the thickness of superposed materials' 
approaches 1.5 km most craters are obliterated. Hence, 
thick lenses of material within deeper basins cannot 
be measured by a crater geometric technique. 

The accuracy or reliability of any map is depen- 
dent in part on the spatial distribution of the data 
points. A point distribution coefficient based on the 
nearest neighbor method is used to determine how 
representative the distribution is of a population 
(Morrison, 1970). The coefficient is calculated by 
the equation: 

R = D (o) /D (e) i D (o) = (d ( i ) and D(e) ='{a/N 

N • 2 

where: D(o) is the mean point distance of the observed 

distribution 


D(e) is the mean point distances of a random 
distribution 

d(i) is the distance from any point to its 
nearest neighbor 
. A is the area 

N is the number of points. 

The point distribution coefficient ranges from 
0.0, when all points are clustered at the same location. 



to 2. 15, when the points have their mazimum spacing 
in a regular hexagonal pattern. The coefficients can 
be classed into the following types of distributions: 


COEFFICIENT 

TYPE 

0.00 - 

0.90 

Clustered 

to random 

0.91 - 

1.25 

Random 


1.26 - 

2.15 

Random to 

uniform 


Greatest confidence is placed in data with, coefficients 
above 1.25 r and little confidence is placed in 
coeffients below 0. SO. 

The overall point .distribution coeffient of the 
western maria is 1.2 with extremes ranging from 0.87 
to 1.39 (Table I) . The distribution is random and 
within the range of useful data. A qualitative check 
on the overall reliability of the data distribution is 
provided by the apparent sensibility of the data in 
correlation with other types of information. 

The relief data at crater rim crests are derived 
from three sources. Where Apollo mapping camera 
stereographic frames exist, topographic maps provide 
coverage with contour intervals of 50 or 100 m. 

Spot elevations at some craters provide elevation 
differences in meters. The error of elevations from 
orbital photogrammetry is approximately 30 m. In 
regions beyond the zone of Apollo mapping frames 



10 

(Pig. 3) relief information is obtained from earth- 
based photogrammetry as incorporated in the Lunar 
Aeronautical Charts or shadow length calculations from 
Orbiter Frames. These data incorporate an average 
error of approximately 100 m. The accuracy of the 
relief aata is thus variable and .less than comparable 
studies of the eastern maria (De Hon and Waskom, 1976) 
which derived the bulk of the data base from Apollo 
Lunar Topographic Orthophotographs. 

A fundamental supposition in estimating the thick- 
ness of basalts requires that the flooded crater 
maintain the essential morphologic and geometric para- 
meters of an .average fresh crater. Three basic 
assumptions exist in regard to crater morphology: 1) 

the crater is reasonably undegraded at the time of 
flooding; 2) the crater follows the morphologic trends 
of craters used to obtain the descriptive function of 
rim height and diameter; and 3) emplacement of mare 
basalt does not significantly alter the original 
crater geometry. These assumptions are the major 
unevaluated factors involved in the thickness 
determinations. The remainder of this section is 
devoted to points which tend to support an a priori 
argument that buried craters retain a fresh crater 
morphology. 



11 


The -residence time of. an unburied crater is 
limited to a restricted time interval. All craters 
used in the estimates formed prior to the beginning 
of Eratosthenian time. Most are Imbrian in age 
(e.g. formed on early Imbrian surfaces and covered 
by late Imbrian basalts) , Some craters are Nectarian 
in age, hence, existed for somewhat longer resident 
times before burial. Burial by basalt emplacement 
inhibits further degradation. It is assumed that 
these craters are, for the most part, preserved with 
little to moderate degradational effects. Many 
Imbrian craters of the highlands maintain crater 
morphologies close to those of Eratosthenian or 
Copernican craters. 

If 

Horz (1978) argues that a closer approximation of 
preserved crater geometry is obtained by using degraded 
crater morphologies. He estimates that these values 
are approximately one-half of the fresh crater values. 
Thus, the resulting isopach map (Horz, 1978) shows 
one-half the thickness obtained for the western maria 
(De Hon, 1978) and for the eastern maria (De Hon and 
Waskom, 1976). I agree, in general terms, with the 
principle that the use of fresh crater equations may 
tend to overestimate the basalt thickness. I oppose 
a nonspecific overall reduction of all thickness 



12 


values because many craters must retain fresh crater 
morphology , it does follow that true thickness may 
be less than that recorded for some areas. If an 
error is introduced , i would prefer the error be 
retained on the thick side and not an unevaluated 
spread of error which allows thickness to be either 
thicker or thinner at any one point. 

Pike’s (1977) equations for crater geometries 

involve averaging rim heights at several points for 
< 

any one crater. A regression line is fitted to the 
average value of a large number of craters. Crater 
thickness estimates use average rim height as well, as 
multiple thickness estimates where possible to minimize 
error introduced by variability in the data. However, 
not all fresh craters follow the established trends. 
Notable exceptions in diameter vs rim height trends 
do occur ( for example Taruntius, Ritter, and Sabine). 
No precaution can exclude craters which do not follow 
the ideal trend. Presumably, the number of these 
craters is relatively small, and the effect of 
inclusion in the data set is minor and offset by nearby 
estimates. The use of these craters gives values 
which are too thick, because as a group all exhibit rim 
heights that are less than normal fresh craters. 



13 


The emplacement of mare material may alter 

original crater morphology. The mechanism by which 

mare material invades crater interiors is not fully 

known. For some flooded craters with incompletely 

exposed rim crest, flooding could be a simple matter 

of overflowing of the lowest elevation of the rim. 

For those craters with flooded interiors but without 

breached rims (e.g, Archimedes), the basalts must be 

emplaced by effusion of basalt from beneath the 
< 

crater floor. It is reasonable to expect floor modi- 
fication (Schultz, 1976a and 1976b) and accompanying 
changes in crater internal geometry. The extent to 
which the exterior rim geometry is altered, if at 
all, is not known. For most "floor-fractured craters" 
which are assumed to have been altered by subsurface 
magmas (Schultz, 1976b), the rim crest is lower than 
that of normal craters; therefore, as a general rule, 
a thickness estimate incorporating these craters may 
be too thick. However, many craters (e.g. Archimedes) 
provide evidence that the rim geometry is essentially 
unchanged by flooding. 

The final factor affecting the accuracy of the 
data is the supposition that the selected buried 
craters are sitting on the pre-mare basement and not 
formed on intrabasalt surfaces. Earth-based and 



14 


lunar orbital spectral studies offer a potential means 

of verification of the stratigraphic horizon of crater 

formation. The rim ejecta of those craters formed on 

* 

the pre-basalt surface may be identified as composed 
of highland-type material (McCord et al. , 1972). Both 
the limiting resolution of the spectral techniques 
and the small numbers of buried craters studied limit 
the contribution of this data to the study at this time. 
Nevertheless a few identified pre-mare craters in 
Oceanus Procellarum provide tie points for the larger 
framework of unverified data. 

Further limitation on the estimates of thickness 
of basalt is imposed by those post-mare (Eratosthenian 
and Copernican) craters which have been excavated 
through the basalt to eject pre-mare terra materials. 
These craters may be identified by the highland 
component of their ejecta (McCord et al. , 1972? 

Andre et al . , 1979). Resolution and dilution effects 
severely limit the number of craters presently 
identified. As an example , the crater Kepler was 
originally classified as a post-mare crater that did not 
exhibit evidence of excavation beneath the basalts? 
hence, the basalts would have had to be thicker than the 
true depth of excavation (approx. 2km, McCord et al. , 
1972). Recently obtained data establishes the 


presence 



15 


of highland material in the rim ejecta (Pieters, 1977); 
hence, the basalt thickness is less than 2km. Post- 
mare crater spectra will be of major importance when 
the resolution allows, identification of highland 
components in the smallest craters to penetrate the 
basalts. Orbital x-ray flourescent spectra have 
established the presence of highland components in 
several post-mare craters in the eastern maria (Andre 
et al . , 1979). Gamma ray spectra have been used to 
identify non-mare ejecta from Lambert and Timocharis in 
South Imbrium (Metzger et al. , 1979) . 

It is hoped that improved resolution spectral 
reflectance studies of pre- and post-mare craters will 
allow selection and final verification of most craters 
used in the construction of the isopach map. A 
qualitative argument for the essential accuracy is 
demonstrated by consistent trends of data and correlations 
of the isopach data with other types of physiographic 
data. In some mare regions, buried craters of various 
sizes are identified within a single locality. Often 
these craters give inconsistent thickness estimates. 

Small buried craters are most likely resting on intra- 
basalt surfaces while the larger craters are resting on 
the basin floor. Hence, large craters generally give 
more consistent results. 



MAP PRODUCTS 


WESTERN MARE PROVINCES 

The western maria are subdivided into seven 
regions (Fig, 5), each of which is characterized by 
one type of dominant subsurface structure or related 
structures. The surface morphology of each of these 
regions is sufficiently distinct to allow description 
of each subdivision in terms of surface characteris- 
tics. The provinces are similiar to those of Hackman 
and Mason (1961). The mare provinces are used as 
convenient regional subdivisions for describing the 
results of this study. The seven provinces are 
defined as follows: 

Imbrium Basin-The Imbrium basin is defined by a 
ring of mountains which rise as much as 5000 m above 
the mare surface. The basin ring is composed of 
Montes Carptus , Montes Apenninus, Monte Caucasus, 
Montes Alpes, and Montes Jura. The ring is breeched 
in three places; in the south between the Carpatus 
and Apenninus sections, in the east opening to Mare 
Serenitatus between the Apenninus and Caucasus 
sections, and on the west (almost a quarter of the 
basin circumference) between the Jura and Carpatus 
sections. Sinus Iridum forms an embayment on the 


16 



17 


northwestern rim. The 1300 km diameter Imbrium 
basin is the largest circular basin on the lunar 
surface and one of the youngest multi-ringed basins. 

Northern Procellarum - The rather featureless 
mare plain of Northern Procellarum contains the 
large crater remnants of Struve, Eddington, and 
Russel to the southwest. The Rumker Hills are the 
most prominent features near the center of the 
region. 

Eastern Procellarum - This region is west of the 
Imbrium basin adjacent to the largest segment of 
breeched Imbrium rim. The mare plain contains the 
Aristarchus Plateau and Montes Harbinger to the 
north and extends south to the Milichus Domes. 

Western Procellarum - The' Western Procellarum 
province extends along the western reaches of 
Oceanus Procellarum from south of Struve to north of 
Mare Humorum. This region includes the Marius Domes, 
the buried crater Flamsteed P, and a prominent linear 
trend of mare ridges. 

Central Lowlands - This region consists of the 
mare lowlands south of Mare Imbrium. Several diverse 
and independent basins are included in this composite 
of lowlands. The lowlands include- Sinus Aestuum, Sinus 
Insularum Mare Vaporum, Sinus Medii, and’ Mare Cognitum. 



Nubium Basins - Mare Hubium is a composite basin 
east of Mare Humorum and south, of Mare Cognitum. 

The region includes Palus Epidemiarum to the south 
and is -limited by the Central Highlands to the east. 

Mare Humorum - This small province consists of 
the circular disc of mare plains in. the Humorum 
basin. The basalt plains are surrounded by an 
ill-defined ring of mountains or rugged terrain 
which is breeched on the east and northeast. 



ISOPACH MAP 


The thickness of basalts in the western maria are 
depicted with an isopach interval of 250 m (Pig, 4). 

The resolution over two-thirds of the map is sufficient 
to portray significant thickness variations over a 
5°area and most lensing occurs over a range of 5°to 10° 
or larger. The average thickness of basalts in the 
western maria is approximately 400m which represents 
an approximate volume of material on the order of 
1.5 x 10 6 km 3 . 

The major features of the isopach map are 
thickening discoidal and prismatoidal deposits of 
basalts and notable regions of basalt thinning. The 
major areas of discoidal lenses correspond to identified 
large circular basins such as Humorum, Cognitum, and 
Imbrium. ' The maximum thickness of basalt in the 
younger circular basin is not known because of the lack 
of preserved buried craters, which generally indicates 
a thickness in excess of 1500m. Other lenses correspond 
to regions which have been suggested as basins based 
on incomplete surface features. At least one new 
buried basin is revealed by the thickening disc in the 
Eastern Procellarum Province (23N;59W). 



Attempts to produce, an isopach map of the Imbrium 
basm basalt have been .frustrated. by the limited number 
of data points and computer contouring resulting in 
a portrayal of an irregular lens thickening. only 
slightly toward, the center and many local irregularities 
The final model of the basalts, in the Imbrium basin 
(Fig, 4) is constructed using auxiliary data input as 
a constraint to the isopach form, A two ringed, 
nested crater is portrayed based on the location of 
mare ridges and isolated massifs (De Hon, 1979) , 

Northern Procellarum is lacking in sufficient 
data for a well defined picture of its probable 
basalt distribution. The spotty data suggest that the 
region as a whole is covered by relatively thin basalts 
with local irregularities. The existence of a 700 km 
diameter basin at 27°N;720W in the region of Struve 
(Howard et al. ; 1974 ; Scott et al., 1977) is not 
apparent m the thickness map (Figs. 4 and 5), if a 

basin did exist in this region it is degraded beyond 
present recognition. 

The amount of information for the remainder of the 
map (Fig. 4) allows the isopach map to be constructed 
with a higher degree of confidence. The East 
Procellarum Province contains a relatively thick lens 
of material adjacent to the large gap in the southwest 



21 


rim of Imbrium. The Western Procellarum Province has 
the form of a long irregular prism of basalt reaching 
1000*'1500 to thick in at least three centers of 
accumulation. The composite nature of the Central 
Lowlands is evident by thick lenses in Mare Cognitum 
and Sinus Aestuum contrasted with thinning of basalts 
southwest of Montes Carpatus and in the Fra Mauro 
region. Mare Humorum is a simple discoid lense, but 
the Nubium Province is more complex (De Hon,. 1977) 



22 


SURFACE TOPOGRAPHY 

Topographic information for the western portion of 
the earth-facing hemisphere is highly variable. The 
Apollo Metric Camera photography and laser altimetry 
is limited to narrow strips crossing southern Imbrium- 
Northern Procellarum and Mare Cognitum-Southern 
Oceanus Procellarum. All other information has been 
acquired by Orbiter spacecraft or photogrammetry of 
earth-based* telescopic photographs. The LAC topographic 
maps provide useful relief data, .but are of question- 
able value for regional, information. As a first 
approximation of possible regional correlation of 
surface topography and mare thickness , comparison is 
made with the 12th degree harmonic topographic model 
(Bills and Ferrari, 1977) . 

The model topography is portrayed with a 500 m 
contour interval and a resolution of approximately 
10 degrees (Bills and Ferrari, 1977). The resulting 
map (Fig. 6) resolves most of the major regional 
scale topographic variations within the area of 
investigation, but averaging of high frequency data 
within large sampling areas does lose 2-4 km from the 
maximum and minimum elevation of the surface in the 
region of Mare Imbrium. The topographic model does 



deplict the troughing along the axis of western 
Oceanus Procellarum and a broad swell in the Central 
Lowland Province, Unfortunately, Mare Humorum and the 
.Nubium region are not readily distinguishable from 
the surrounding regions,. The terra regions, however, 
are shown as areas of higher terrain. 

Previous attempts to portray regional or global 
lunar surface topography vary considerably in detail 
and absolute elevations (for example: Baldwin, 1963; 

Army Map Service, 1960; ACIC LAC Topographic Maps) 
but similar trends are common to most maps. The 
circular basins exhibit depressed centers; the Western 
Mare Province is a linear topographic depression; and 
the central highlands are an elevated region. It is 
significant that where more detailed topography is 
available, thick lenses of basalt are almost 
invariably marked by a topographic low. 



SUBSURFACE TOPOGRAPHY 


As a first approximation of the pre-basalt 
paleotopography the isopac-h map (Fig, 4) is subtracted 
from the 12^ degree harmonic topographic map (Fig, 6). 
The results, contoured with a 500m contour interval, 
portray a model of the surface topography with the 
basalt removed (Fig. 7), If the topographic model is 
accurate and it isostatic compensation has not taken 
place in response to the basalt, then the map portrays 
the lunar surface prior to the emplacement of the 
basalt. Unfortunately, any short-comings of the 
topographic model are included in the subsurface 
model. 

Nevertheless, the "shoreline" is reasonably • 
represented by the -1500m contour line. Most of the 
identified basins are included on the pale cPt opographic 
model. Inasmuch as there is an approximate overall 
correlation of basalt thickness .and surface elevation, 
the effect of the subtraction is to accentuate 
depressions . 

The paleotopographic model (Fig, 7) is 
characterized by depressed surface -with maximum relief 
in excess of 5500 m from mid-Imbrium basin to the 
Central Highland bulge. Individual basins are 



portrayed as roughly subcircular depressions 1000 m 
to 2500 m deep. Actual depths are probably somewhat 
greater since the harmonic model tends to lose 
maximum and minimum elevations CBills and Ferrari , 
1977) , 

On a regional scale, most of the Procellarum 
basin forms a large arcuate trough which runs from 
the northwestern edge of the map south and southeast 
in to the Nubium basin. The trough is a significant 
constituent of the pre-flooded terrain, but its 
genesis is largely problematical. The trough may 
represent a chance alinement of basin-sized impacts, 
or some sort of a mega-circum-Irabrium trough. 

The Imbrium basin dominates the northeastern 
portion of the map. The basin is revealed as a 
discontinous ring of raised topography (above -2000m 
elevation) surrounding a broad shelf (-2500 to -3000m 
level). A relatively deep inner basin (elevations 
less than -3000 m) occupies the basin center. The 
scale, contour interval, and generalized nature of 
the map does not allow adequate representation of 
the prominent inner ring of the basin between the 
shelf and inner basin. 



SYNTHESIS 


SURFACE FEATURES- 

A sketch map of the major mare surface features 
is presented for comparison with the isopach map and 
related map products. This map (Fig. 8) shows the 
location of the major rilles, mare ridges, and domes. 
The data base for the surface maps is a composite 
and simplification of the surface features depicted 
on the 1:1,000,000 geologic maps of the moon and the 
1:2,500,000 synthesis map of Scott et. al . (1975). 

A close correlation between the location of mare 
surface structures and thickness trends was 
demonstrated for the eastern maria (De Hon and Waskom, 
1976) . The more extensive basalt flooding and the 
scarcity of interbasinal highland materials make 
this an even more significant study in the western 
maria. Many of the major surface trends do show 
significant correlations with the isopach map and/or 
subsurface model. Some features, which do not show 
the expected correlations, may point to unsatisfactory 
models of basalt thickness or to incompletely under- 
stood control mechanisms. 

Arcuate rilles invariably are located in regions 
of thin basalts. They are usually close to the edge 


26 



27 


of the basin and parallel or subparallel to it. Most 
are confined to the basalts, but unlike the rilles 
of the eastern maria many rilles transect both mare 
material and highland material, Notable examples of 
this class are the rilles of the Rimae Hippalus set 
which are outside of and concentric to the southeast 
raised rim of Mare Humorum, The rilles belonging to 
the Rimae Bradley and Fresnel sets in Mare Imbrium 
are parallel to the Montes Apenninus Scarp, but they 
transect pre-mare material. 

There is a concensus of opinion that mare rilles 
are graben developed in response to tension at the 
edges of the basins (Baldwin, 1963; Quaide,' 1965; 
Smith, 1966; McGill, 1971) . The preferential 
location in thin basalts (Fig, 8) is consistent with 
an origin requiring shallow stresses decoupled from 
the underlying basement (De Hon and Waskom, 1976) . 
Specifically, the intersection of faults bounding 
a graben marks the contact between crustal materials 
of contrasting properties (McGill, 1978). Recent 
studies of the probable depth to the discontinuity 
based on fault intersections yields estimates 
ranging from 1,5 to 3 km (Golombek, 1978), There 
appears to be no significant difference between mare 
or terra rilles. Hence, the decoulement is probably 



28 


at the base of the lunar niegabreccia (Golombek, 1978) 
and not at the contact between mare basalt and 
megabreccia. - The tensional stress field may be 
imposed by basalt consolidation and volume -reduction 
(Swanson and Peterson,. 1972; pe. Hon and Waskom, 1976) 
or subsidence of the basin in response to the load 
imposed by the basalt (Melosh, 1978). 

Sinuous rilles generally occur over thin basalts 
near the periphery of thickening wedges, often 
associated with regions of domes. Most sinuous rilles 
follow the slope of the surface. Hence they appear 
to originate in regions of thin basalts and run 
into topographic lows over thicker basalts. 

Domes are largely concentrated in a few broad 
regions also characterized by thin basalts over 
subsurface highs. The largest concentration of domes 
occurs in the Marius Hills region of western 
Procellarum on the edge of a' thick lens of basalt. 

The Milichus domes occur southeast of Mare Imbrium 
on the crest of a broad subsurface ridge which 
divides the East Procellarum and Central Lowland 
Provinces. The Aristarchus Plateau is located at the 
edge of the shallow North Procellarum shelf adjacent 



29 


to the East Procellarum basin. Domes of Mons Rumker 
are located in apparently thin basalts of the 
Northern Procellarum Province, 

Sinuous rilles and domes are in part closely 
correlated in time and space with each other and 
both are correlated to regions of thin mare basalts 
or topographic highs of the basement floor. Sinus 
rilles appear to be associated with the latest source 
vents for young mare basalts (Shaber, 1973). whether 
domes and sinuous rilles are products of escaped 
lava from shallow local residual magma pockets 
(De Hon and Waskom 1976) or an indication of deeper 
crustal conduits to the surface is unresolved at 
this time. Whitford-Stark and Head (1977) suggest 
that the Marius and Aristarchus volcanic complexes 
may be the source vents for much of the central 
Procellarum mare fill. 

Mare ridges exhibit preferential locations 
similar to the controls observed in the eastern 
maria (De Hon and Waskom, 1976). Concentric trends 
within the well-defined circular basins of Humorum 
and Imbrium are apparent. Other partial arcs and 
circular patterns are observed associated with thick 
lenses of basalts which mark the location of totally 



30 


flooded basins such as the west Nubium basin and those 
of western Procellarum, The western Procellarum 
Province is characterized by a broad trough with a 
linear trend of mare ridges on either side of the 
trough axis. For the most part, mare ridges are 
located along zones of changing basalt thickness 
(e.g. areas of maximum paleoslopej adjacent to the 
thickest lenses. 

There is less concensus as to the origin of 

mare ridges. Hypothesis of mare ridge origin include: 

(1) draped subjacent topography (Baldwin, 1963; 

* 

Maxwell et al. 1975); (2) intrusive or extrusive 
volcanism including auto^j.ntrusion (Whitaker, 1966; 
Strom, 1972; Hodges, 1973; Scott et al., 1975); 

(3) tectonic deformation resulting from isostatic 
subsidence or mascon loading (Bryan, 1973; Melosh, 
1978); and (4) tectonic formation due to crustal 
compression accompanied by thrusting (Shaber, 1973; 
Howard and Muehlberger, 1973) . A combination of 
processes is favored by Tjia (1970), Hartman and 
Wood (1971), Colton et al. (1972) and De Hon and 
Waskom (1976) . Many of the suggested origins are 
not mutually exclusive. The preferential location 
of mare ridges in zones of changing basalt thickness 



31 


(Fig. 4 and 8) provides at least partial control 
for subsiquent hypotheses of origin. Mare ridges 
are largely surface features (probably compressional) 
localized by the configuration of the basin floor 
(fig, 7), 



32 


CIRCULAR BASINS AND RELATED STRUCTURES 

Oceanus Procellarum and. the western maria are 
largely composed of superposed or contiguous circular 
basins (Fig, 9], The youngest basins are readily 
identifiable by a large number of basin related 
features such as complete to partial arcuate raised 
rims, relatively thick basalt lens, concentric ridge 
patterns, and positive gravity anamolies. Older 
basins retain progressively less features and 
identification eventually becomes problematical. 

Most of the larger basins probably have inner 
rings and a nested-crater structure. Humorum, 

Nubium, and the northern-most West Procellarum 
basin exhibit partial arcs of mare ridges which 
occupy the inner ring location. However, the scale 
of isopach construction and data spacing does not 
allow reconstruction of the complex inner structure 
(Fig. 4). Most basins are portrayed as simple, 
thickening discs. Table II lists 14 probable basins 
of the region. Mare Imbrium occupies the largest 
and youngest of the basins. Mare Humorum, 

Mare \aporum and Sinus Medii are among those which 
still retain an identifiable circular outline. 

Mare Cognitum is probably a composite of two small 



basins with small segments of the rim preserved as 
Montes Riphaeus and isolated terra patches around 
the periphery, Mare Nubium is a composite structure 
comprised of adjacent basins (De Hon, 1977) . 

Other regions are chosen as probable basins 
on the basis of incomplete arcs of terra contact, 
mare ridge patterns, or thickening lenses of basalt. 
The isopach map (Fig. 4) • • confirms or 

lends support to the presence of basins in Sinus 
Aestuum, the Flamsteed region (6°S;43°W), and 
northern West Procellarum (8°N;59°W). The existance 
of a basin in the region of Struve (26°N;72°W) is not 
confirmed by a measurably thick lens of basalt; 
however, the scalloped nature of the mare- terra 
contact and an arcuate mare ridge pattern are 
suggestive of a basin. 

The identification of a relatively thick lens 
of basalt in the East Procellarum region (23°N;36°-W) 
points, to a probable pre-Imbrian basin in this 
region. The existence of the East Procellarum basin 
helps explain the otherwise enigmatic break in the 
southwest rim of the Imbrium basin. 

Circum-basinal troughs (De Hon and Waskom, 

1976) are not abundant, but at least partial arcs 
of lowlands may be observed around some of the 



basins [Fig. 4 and 9) . Pains Epidemxarum. is flooded 
terrain at the intersection of a circum-Humorum and 
circum-Nubium trough (De Hon f 1977).. Short 
fragments of troughs may be recognized around the 
West Procellarum basins. Northern Procellarum — 
connects with Mare Figoris which forms a well- 
defined arc of lowland circumferential to Mare 


Imbrium. 



35 


LUNAR GRAVITY AND BASALT THICKNESS 

No attempt is made in this study to quanitatively 
correlate gravity and basalt thickness. However, 

‘some qualitative comparisons are significant. Major 
mascons (Muller and Sjogren, 1968) are associated 
with the youngest, large circular basins. Mare 
Imbrium exhibits a 180 mgal positive anamoly, and 
Mare Humorum exhibits a 160 mgal anamoly. A small, 

+20 mgal linear trend in western Procellarum is 
coincident with the trough of thick basalts. Sinus 
Medii and the Sinus Aestruum-Stadius region are 
both characterized by +20 mgal anamolies. 

From a first assumption of a single dense 
buried mass (Stripe, 1968; Urey, 1968) to a more 
complex model of relief at the lunar mantle (Wise 
and Yeats, 1970; Philips et ad..,' 1972) a 
comprehensive model of lunar interior structure and 
gravity is gradually emerging.- The mare basalts 
are largely superisostatic loads imposed on an 
otherwise isostatrcally compensated surface (Bowin 
et al., 1975). The observed gravity variations are 
the combined results of relief of the lunar mantle 
formed in response to early topographic irregularities 



36 

(such as basins) plus the added effects of an 
uncompensated load imposed by basalts .emplaced on 
a rigid surface, 

Thurber and Solomon (1978) have produced 
several models of crustal structure to match lunar 
gravity. One model (Fig. 10) involves superisostatic 
basalts. While this model does not conform in detail 
to the basalt thicknesses derived in this study, it 
does agree in magnitude and overall characteristics. 
Apparent discrepancies between the photomorphometric 
and gravity-derived thickness may point to regions 
of partial isostatic compensation after basalt 
emplacement. 

The extent to which isostasy has modified the 
surface of the basalts remains an unsettled point. 

The pre-basalt basins were almost certainly brought 
into iso static equilibrium (Thurber and Solomon, 

1978). Whether post mare-fill isostatic adjustments 
have occurred, and to what extent, is a significant 
factor in attempting to reach agreement between 
photogeologic and geophysical models. Some 
isostactic subsidence of the basalts is required 
by various models of mare ridges and rilles (Hov/ard 
and Muehlberger , 1973? Thurber and Solomon, 1978). 



37 


Isostatic subsidence appears t.o' be the most logical 
mechanism to form the concentric rille pattern 
southwest of Mare Humorum (pe Hon, 1977), Not all 
surface subsidence need be isostatic t rather some 
subsidence may be the result of volume reduction 
of lavas by outgasing and consolidation (Cruikshank 
et al , t 1973; De Hon and Waskom, 1976). 



SUMMARY AND DISCUSSION 


The basalts of the western maria occupy large 
circular basins and surrounding lowlands associated 
with the basins. While Mare Imbrium and Mare Humorum 
are the only well-preserved circular basins, isopach 
mapping reveals that even the vast expanse of basalt ■ 
plains of Oceanus Procellarum mantles a composite 
structure of several coalescing basins. The 
thickest basalts are found in Mare Imbrium, Mare 
Humorum, and the western part of Oceanus Procellarum. 
Relatively thin basalts are found south and southwest 
of Mare Imbrium in the Central Lowland region and in 
Northern Procellarum. The average thickness and 
total volume of basalts in the western basins is 
greater than that of the eastern maria. 

For the most part, the general conclusions of 
the earlier studies of the eastern maria are 
confirmed in the western maria. These conclusions 
are as follows: 

1. The basalts are relatively thin, averaging 
approximately 400m thick, with thickening discs 
associated with major basins. 

2. Younger basins contain thicker accumulations 
of basalt than older basins. 


38 



39 


3. Xsopach mapping confirms the existence of 
previously proposed buried basins and has identified 
regions of probable unidentified basins. 

4. Rilles tend to be located in relatively 
shallow mare basalts parallel or subparallel to zones 
of equal thickness. 

5. Mare ridges tend to be located at the site 
of buried topography or in regions of maximum 
variation in basalt thickness (e.g. transition 

t 

between thin basalts and thick lenses) , 

6. Domes tend to be located on relatively 
thin basalts associated with regional rises of the 
basement topography. 

7. Regional variations of basalt surface 
elevations mimic (at a lesser relief) subsurface 
topography. 

8. Positive gravity anomalies tend to be 
associated with the thickest basalt lenses in 
circular basins, but not all probable basins 
exhibit gravity anomalies. 

A few notable and possibly significant 
exceptions to these generalizations are found in 
the western maria. The exceptions may provide as 



40 


much insight into the structural regime and history 
of the maria as the general trends. These exceptions 
are as follows: 

1* Nany arcuate rilles are not restricted 
to mare basalts, but cut highland and mare material 
with equal development. These rilles must be 
related to regional tensional stresses affecting 
basement material's. 

2. The mare ridge pattern in Northern 
Procellarum is not associated with an identified 
thickening lens. 

Any comparison of the eastern and western 
maria draws attention to a distinct character in the 
distribution of mare materials. The eastern maria 
comprise less surface area and the composite nature 
is apparent. The eastern basalts are more 
discontinuous being restricted to basins and 
lowlands with only minor overflow connecting the 
basalt surface. On the other hand, the western 
maria are connected by an almost continuous surface 
of basalts over a much larger area. The prime 
distinction between the eastern and western maria 
appears to be one of basalt volumes. The western 
basins and adjacent terrains are submerged by a 
thicker mantle of basalts. 



The mare basalts are emplaced in low lying 
terrain which forms an inverted yoke around the 
Central Highland earth-'directed bulge. Maximum 
volumes of basalt are deposited west of the Central 
Highlands flooding the terrain to a greater extent. 
Surface structures such as rilles and ridges are 
restricted to the mare basalt in the eastern maria 
and can be interpreted as primarily formed by 
stresses internal to the basalts. Similar 

t 

structures of the western maria transect mare-basalts 
and highland material; hence they must be related 
to less restrictive stresses. The western maria 
and related surface structure may reflect the 
probability of a greater degree of isostatic response 
to a larger surface loading by the thicker basalts. 



ACKNOWLEDGEMENTS 


This work was supported by NASA Grant NSG 7216 
.at the University of Arkansas at Monticello and NASA 
Grant NSG 7^20 at Northeast Louisiana University, The 
help of Mrs, Nell Cooley and Mrs, Janice K, Ledoux is 
gratefully acknowledged, Mr, John B, Sharp, Miss M, 
Denise Young, and Miss. Susan Glenn also provided 
valuable assistance. 


42 



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48 


Fig. 


Fig. 


Fig. 3 


Fig. h 


Fig. ^ 


Fig. 6 


Figure Captions 

L. External rim height as a function of crater 
diameter determined from fresh appearing lunar 
craters (Pike, 1977). 

1. Method for determining hasalt thickness in the 
vicinity of a flooded crater. Estimate ignores 
local variation in the center of the crater 
and the rim. 

• Location of interior data points used to construct 
the isopach map. Numbers in parentheses are 
arbitrary estimates in thick lenses which allow 
competion of the contouring program, 

Isopach map of the western mare basalt, 

Isopach interval is 250m. Regions of insufficient 
data are shown with diagonal ruled pattern. 

, . Provinces of the lunar western maria as defined 
in this paper. 

. Model topography of the western portion of the 
moon. 12th degree harmonic topographic model 
after Bills and Ferrari (1977) Contour interval 
is 500 m. 



49 


Fig. 7. Model paleotopography derived by subtracting the 
isopach map from model topography. Contour 
interval is 500 m. 

Fig. 8. Sketch map of major mare surface features. 

Compare surface trends with figs, 6 and 7 for 
possible spatial correlations. 

Fig. 9. Location of probable impact basins in the 

western maria. Dots mark approximate location 
of outer ring and inner ring if identified. 

Arcs of depressed terrain outside the circular 
basins are indicated by dotted line with arrows. 

Fig. 10. Basalt thickness map derived on the assumption 
that all superisostatic loading in mare basins 
can be attributed to mare basalt, (Thurber and 
Solomon, 1978) . Isopach interval is 500 m. 



TABLE 

Province 

I. Point Distribution Coefficient by Mare Province 

Area (deg.^ )/ 

Number of Points No. of Points Distribution Coefficient 

N. Procellarum 

37 

29.4 

1.03 

E. Procellarum 

8 

50.0 

0.90 

W. Procellarum 

61 

32.7 

0.87 

Imbrium 

54 

29.8 

1.06 

Central Lowland 

42 

14.9 

1.09 

Humorum 

27 

11.1 

0.91 

Nubium 

48 

11.5 

1. 03 

(Palus Epidemiarum) 

16 

4.7 

1.39 

"Whole Map" 

29.4- 

13.7 

1. 20 



TABLE II 


Basin 

Imbrium 

Stadius-Aestuum 

E. Procellarum 

NW. Procellarum 

Near Struve 

SW. Procellarum 

Humorum 

W. Nubium 

E. Nubium 

NW. Cognitum 

Iridium 

Medii 

Vaporum 

SE. Cognitum 


Location 


Lat. (°) 

Long. (° ) 

36N 

16W 

12N 

13W 

23N 

3 6W 

8N 

59W 

27N 

72W 

6S 

43W 

24S 

39W 

24S 

22W 

23S 

low 

10S 

24 W 

45N 

31W 

0 

IE 

13N 

3E 

12S 

21W 


Probable Circular Basins 


Diameter (km) 


Outer Ring 

1350 

720 

710 

700 

675 

550 

460 

410 

370 

290 

260 

250 

240 


Inner Ring 

620 

490 

200 

315 

270 
' 200 
180 


200 




2.5- 

in km 

1.0- 

_e 

ce 

.5- 

IE 

c r> 

<D 

X 

' .25 •* 

E 

ir 

,i - 


1 

LO 

O 

* 



R^ = Rim Height 

R @ ~ Exposed Rim Height 

T = Thickness of Mare 


Figure 2 



LOCATION OF INTERIOR DATA POINTS 







1S0PACH MAP OF WESTERN MARE BASALTS 


Isnpjcfe Inteml is 250 Meiers 





Figure 5 








MODEL TOPOGRAPHY 

ConloBi Inleiial is 500 tftteis 













Figure 8 





LOCATION GF PROBABLE IMPACT BASIHS 






Figure 10 


i 


3i